Add support for disabling reloader

**Is your feature request related to a problem? Please describe.**

I'm an Apache Arrow developer. We're using this product for testing our GCS filesystem implementation.

We run this product by `python3 -m testbench`. This launches multiple processes for reloader. To shutdown these processes, we need to (1) shutdown the main process gracefully (the main process shutdowns other processes) or (2) terminate all related processes by ourselves.

Our test uses Boost.Process https://www.boost.org/libs/process to launch this product. We're using (2) because Boost.Process can't do (1) on Windows. I don't know why but (1) with Boost.Process shutdowns the launcher process (the main test process in our case) too.

We're migrating Boost.Process v2 API https://github.com/apache/arrow/pull/43766 because Boost.Process v1 API is deprecated since Boost 1.86.0. Unfortunately, we can't use (2) with Boost.Process v2 API because it doesn't support process group: https://github.com/boostorg/process/issues/259

If this product doesn't use multiple processes, we don't need to use (2). We can just need to terminate the main process. It can simplify our test.

**Describe the solution you'd like**

How about adding a new `--no-use-reloader` option that disables reloader something like the following?
If we disable reloader, `python3 -m testbench` launches only one process.

```diff
diff --git a/testbench/rest_server.py b/testbench/rest_server.py
index 9490f24..579b349 100644
--- a/testbench/rest_server.py
+++ b/testbench/rest_server.py
@@ -1174,12 +1174,14 @@ def _main():
description="A testbench for the GCS client libraries"
)
parser.add_argument("--port", default=0, type=int)
+ parser.add_argument("--use-reloader", default=True,
+ action=argparse.BooleanOptionalAction)
args = parser.parse_args()
serving.run_simple(
"localhost",
port=args.port,
application=_run(),
- use_reloader=True,
+ use_reloader=args.use_reloader,
threaded=True,
)

```

If we keep the default value of it as `True`, we can still use reloader by default.

**Describe alternatives you've considered**

Something that only uses one process.

**Additional context**

None.